This is an excerpt from Fitness Illustrated by Brian J. Sharkey.
Don't Call It Cardio!
Back in the 1950s, our knowledge of fitness was limited to the effect of training on the heart. Training led to a reduction in the resting and exercise heart rates, so it was called cardiovascular fitness. As we began to understand the effect of training on oxygen intake and oxygen transport, it came to be called cardiorespiratory fitness. Then, in 1967, Dr. John Holloszy published a landmark study of the effects of training on skeletal muscle fibers (i.e., the cells found inside such muscles as your biceps, quadriceps, and hamstrings). The study showed that training doubled oxidative enzymes and the trained muscle's ability to use oxygen. In other words, the more a person trains, the better his or her muscles use oxygen. From that point onward, we have defined fitness as a person's maximal ability to take in, transport, and use oxygen.
The term cardio does describe one part of aerobic fitness, but it ignores the important effects of training on muscle fibers. Once you understand the full picture, you can design your fitness training program the right way. You can't become aerobically fit simply by raising your heart rate; if you could, then the mere elevation of heart rate from receiving a scare would improve fitness. Nor can you become fit by using any old variety of muscles to simply raise heart rate. Instead, you need to systematically use your large muscle groups in order to get oxygen circulating to the muscles used in the activity (running, cycling, etc.). Heart rate is sometimes used as a measure of how much oxygen you are using in order to gauge exercise intensity. But skeletal muscle (in the muscles used in the activity) is the target of training. The role of your heart is to supply the working muscles with oxygen and energy.
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